If you want to render an object and overlay it onto a photo background, use the "kernel"->"alphachannel" -> enable. Then render and save the resulting image to PNG. Then load into Photoshop/GIMP and overlay it onto your photo.
One thing to keep in mind is that Octane doesn't support screen mapping a texture.
To make an object with a texture which is not affected by any lighting, use a diffuse material with diffuse reflection 0.0, then add an emission node and put your texture into the efficiency/texture slot.